1. Field of the Invention
This invention relates to micromechanical structures having a capacitive transducer gap filled with a dielectric and method of making same.
2. Background Art
The following non-patent references are referenced herein:                [1] J. Wang et al., “1.51-GHz Nanocrystalline Diamond Micromechanical Disk Resonator With Material-Mismatched Isolating Support” TECHNICAL DIGEST, IEEE INT. MICRO ELECTRO MECHANICAL SYSTEMS, CONF., Maastricht, The Netherlands, Jan. 2004, pp. 641-644.        [2] S.-S. Li et al., “Micromechanical ‘Hollow-Disk’ Ring Resonators,” TECHNICAL DIGEST, IEEE INT. MICRO ELECTRO MECHANICAL SYSTEMS CONF., Maastricht, The Netherlands, January 2004, pp. 821-824.        [3] J. Wang et al., “1.156-GHz Self-Aligned Vibrating Micromechanical Disk Resonator,” IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, Vol. 51, No. 12, pp. 1607-1628, December 2004.        [4] C. T.-C. Nguyen, “Vibrating RF MEMS for Next Generation Wireless Applications,” PROCEEDINGS, IEEE CUSTOM INTEGRATED CIRCUITS CONF., Orlando, Fla., October 2004, pp. 257-264.        [5] V. Kaajakari et al., “Stability of Wafer Level Vacuum Encapsulated Single-Crystal Silicon Resonators,” DIGEST OF TECHNICAL PAPERS, INT. CONF. ON SOLID-STATE SENSORS, ACTUATORS, AND MICROSYSTEMS (TRANSDUCERS '05), Seoul, Korea, June 2005, pp. 916-919.        [6] B. Kim et al., “Frequency Stability of Wafer-Scale Encapsulated MEMS Resonators,” DIGEST OF TECHNICAL PAPERS, INT. CONF. ON SOLID-STATE SENSORS, ACTUATORS AND MICROSYSTEMS (TRANSDUCERS '05), Seoul, Korea, June 2005, pp. 1965-1968.        [7] W.-T. Hsu et al., “Stiffness-Compensated Temperature-Insensitive Micromechanical Resonators,” TECHNICAL DIGEST, IEEE INT. MICROELECTRO MECHANICAL SYSTEMS CONF., Las Vegas, Nev., January 2002, pp. 731-734.        [8] Y.-W. Lin et al., “Series-Resonant VHF Micromechanical Resonator Reference Oscillators,” IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 39, No. 12, pp. 2477-2491, December 2004.        [9] G. Piazza et al., “Low Motional Resistance Ring-Shaped Contour-Mode Aluminum Nitride Piezoelectric Micromechanical Resonators for UHF Applications,” TECHNICAL DIGEST, IEEE INT. MICRO ELECTRO MECHANICAL SYSTEMS CONF., Miami Beach, Fla., January 2005, pp. 20-23.        [10] S.-S. Li et al., “Self-Switching Vibrating Micromechanical Filter Bank,” PROCEEDINGS, IEEE JOINT INT. FREQUENCY CONTROL/PRECISION TIME & TIME INTERVAL SYMPOSIUM, Vancouver, Canada, August 2005.        [11] A.-C. Wong et al., “Micromechanical Mixer-Filters (“Mixlers”),” JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, Vol. 13, No. 1, pp. 100-112, February 2004.        [12] B. Bircumshaw et al., “The Radial Bulk Annular Resonator:        
Towards a 500Ω RF MEMS Filter,” DIGEST OF TECHNICAL PAPERS, INT. CONF. ON SOLID-STATE SENSORS, ACTUATORS AND MICROSYSTEMS (TRANSDUCERS '03), Boston, Mass., June 2003, pp. 875-878.                [13] Y. Xie et al., “UHF Micromechanical Extensional Wine-Glass Mode Ring Resonators,” TECHNICAL DIGEST, IEEE INTERNATIONAL ELECTRON DEVICES MEETING, Washington D.C., WA, December 2003, pp. 953-956.        [14] M. U. Demirci et al., “Mechanically Corner-Coupled Square Microresonator Array for Reduced Series Motional Resistance,” DIGEST OF TECHNICAL PAPERS, INT. CONF. ON SOLID-STATE SENSORS, ACTUATORS AND MICROSYSTEMS (TRANSDUCERS '03) Boston, Mass., June 2003, pp. 955-958.        [15] S. A. Bhave et al., “Silicon Nitride-on-Silicon Bar Resonator Using Internal Electrostatic Transduction,” DIGEST OF TECHNICAL PAPERS, INT. CONF. ON SOLID-STATE SENSORS, ACTUATORS AND MICROSYSTEMS (TRANSDUCERS '05), Seoul, Korea, June 2005, pp. 2139-2142.        [16] M. Onoe, “Contour Vibrations of Isotropic Circular Plates,” J. ACOUSTICAL SOCIETY OF AMERICA, Vol. 28, No. 6, pp. 1158-1162, November 1956.        
Capacitively transduced vibrating micromechanical resonators have recently been demonstrated with resonant frequencies in the GHz range with Q's larger than 11,000 [1] [2] [3], making them very attractive as on-chip frequency control elements for oscillators and filters in wireless communications. Although solutions now exist to many of the issues that once hindered deployment of these devices in RF front ends [4], including aging [5] [6] and temperature stability [7], the need for high bias voltages to reduce impedances, especially in the VHF and UHF ranges, still remains a troublesome drawback of this technology. For example, in reference oscillator applications, where the impedance of the micromechanical resonator must be low enough to allow oscillation startup, dc-bias voltages on the order of 12 V, much larger than normally permitted in standard integrated circuit (IC) technologies, have been required to attain GSM-compliant phase noise specifications [8]. For off-chip filter applications, impedance needs in the 50-377Ω range (e.g., for antenna matching) are even more challenging, so require even high voltages. Needless to say, a method for attaining low impedance with IC-amenable voltages would be highly desirable.
For this matter, micromechanical resonators using piezoelectric transducers already achieve low impedance, and without the need for bias voltages. Although many such piezoelectric designs suffer the drawback of having frequencies governed primarily by thickness, hence, not CAD definable; new piezoelectric micromechanical resonators that harness the d31 coefficient to allow lateral operation now circumvent this problem [9], making piezoelectric transduction much more attractive. Still, the (so far) higher Q's of capacitively transduced resonators, their allowance for more flexible geometries with CAD-definable frequencies, plus their self-switching capability [10], voltage-controlled reconfigurability [11], better thermal stability [7], and material compatibility with integrated transistor circuits, make them sufficiently more attractive to justify intense research into methods for lowering their impedance while keeping voltages low. Among these, assuming that electrode-to-resonator gap spacings have already been minimized, methods that increase electrode-to-resonator overlap area by either direct geometrical modification [2] [12] [13] or mechanically-coupled arraying [14] have been most successful, albeit at the expense of die area.
The following U.S. patent references are related to the present invention: U.S. Pat. Nos. 6,846,691; 6,985,051; 6,856,217; 2006/0017523; and U.S. Pat. No. 6,628,177.